The present invention relates to a green phosphor for fluorescent display and a method of manufacturing the same. Specifically, the present invention relates to a ZnGa2O4 based green phosphor for fluorescent display having high luminous efficiency and high color purity and a method of manufacturing the same.
A fluorescent display, specifically a field emission display (FED) is a flat panel display working by the same principle with the Braun tube, in which a cathode plate which is a field emission element array panel discharging electron by electric field in place of heat and an anode plate which is a fluorescent panel receiving electrons and emitting light are disposed parallel to each other at a predetermined distance packaged under high vacuum. Sulfide-based phosphors having high luminous efficiency have been generally used in the conventional Braun tube. However, the distance between the cathode plate and the anode plate in the field emission display is small, so that high voltage more over 10 kV such as used in Braun tube causes arcing. Therefore, a low voltage not more than 5 kV is required for the field emission display. Specifically, FED operating with voltage not more than 1 kV has been studied for development all over the world.
When energies of electrons are equal to or lower than 1 kV, the electron penetrating depth is estimated less then 20 nm from a phosphor surface. Therefore, there is a problem that the luminous efficiency and the luminance of the phosphor for FED operating at low voltage are much lower than those of the conventional Braun tube. Also, there is a problem that a surface state of the phosphor greatly influences the luminous efficiency of the phosphor.
Specifically, if the sulfide-based green phosphor, ZnS: Cu, Al, widely used in the conventional Braun tube is used as a phosphor for FED, not only the luminous efficiency are very low at low operating voltage, but also a small amount of sulfur is liberated from the sulfide-based phosphor by irradiating an electron beam during a long time. There is another problem that in an FED panel having a distance of about 1 mm between the cathode plate and the anode plate, an internal space under vacuum between the cathode plate and the anode plate is small and, vacuum level is by liberated sulfur. The degradation of ZnS phosphor during operation will also lead to poisoning of the field emission cathode tips, so that performance of the display is deteriorated. Recently, in order to solve such problems, oxide-based phosphors which have no such problems have been widely studied.
Therefore, the present invention is made in order to solve the conventional problems described above.
An object of the present invention is to provide oxide-based green phosphor for fluorescent display not causing degradation of sulfur during operation but having a high luminous efficiency and high color purity.
Another object of the present invention is to provide a method of manufacturing the green phosphor for fluorescent display in simple and economic manners. The above objects can be accomplished by a green phosphor for fluorescent display having a composition represented by a chemical formula:
xZnO+(2xe2x88x92xxe2x88x92y/2)Ga2O3+yAl2O3:zMn2+
where 0.8xe2x89xa6x less than 1.0; 0 less than yxe2x89xa60.8, and 0 less than zxe2x89xa60.1,
wherein a part of gallium in nonstoichiometric zinc gallate base is substituted for aluminum and Mn2+ is added to the zinc gallate base.
Also, in order to accomplish the above objects, a method of manufacturing a green phosphor for fluorescent display having a composition represented by a chemical formula:
xZnO+(2xe2x88x92xxe2x88x92y/2)Ga2O3+yAl2O3:zMn2+
where 0.8xe2x89xa6x less than 1.0; 0 less than yxe2x89xa60.8, and 0 less than zxe2x89xa60.1, is provided. The method includes the following steps: A first step is of preparing a mixture by mixing uniformly zinc oxide, gallium oxide, aluminum oxide, alcohol and either an aqueous solution of manganese salt or an aqueous suspension of manganese oxide. A second step is of preparing a compound by heating said mixture. And a third step is of reducing said compound by re-heating said compound in a reducing atmosphere.
It is preferable that ZnO is used as said zinc oxide, Ga2O3 is used as said gallium oxide, Al2O3 is used as said aluminum oxide, MnO or MnO2 is used as said manganese oxide, and MnCl2 is used as said manganese salt.
Also, it is preferable that said step of preparing a compound is performed by heating said mixture at a temperature of 1000xc2x0 C. to 1300xc2x0 C. for 4 to 10 hours, and said step of reducing said compound is performed by re-heating said compound in the reducing atmosphere in which a volume ratio of nitrogen and hydrogen is within a range of 100%:0% to 80%:20%, at a temperature of 900xc2x0 C. to 1000xc2x0 C. for 0.5 to 5 hours.
Also, it is still preferable that in said step of preparing a mixture, said mixture has an element ratio of zinc and gallium smaller than 1:2 and 0.05 to 0.15 m/o of Mn2+, said step of preparing a compound is performed by heating said mixture at a temperature of 1100xc2x0 C. to 1300xc2x0 C. for not more than 10 hours, and said step of reducing said compound is performed by re-heating said compound in the reducing atmosphere in which a volume ratio of nitrogen and hydrogen is not more than 100:5, at a temperature of 900xc2x0 C. to 1000xc2x0 C. for about 3 hours.
Luminescence characteristics of Zinc gallate based phosphor can be maximized by optimizing surface compositions, diameters and shapes of the phosphor powders.